Device and method for measuring six degrees of freedom

ABSTRACT

A laser tracker system for measuring six degrees of freedom may include a main optics assembly structured to emit a first laser beam, a pattern projector assembly structured to emit a second laser beam shaped into a two-dimensional pattern, and a target. The target may include a retroreflector and a position sensor assembly. A center of symmetry of the retroreflector may be provided on a different plane than a plane of the position sensor assembly. A method of measuring orientation of a target may include illuminating the target with a laser beam comprising a two-dimensional pattern, recording a position of the two-dimensional pattern on a position sensor assembly to create a measured signature value of the two-dimensional pattern, and calculating an orientation of the target based on the measured signature value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/620,070, filed Nov. 17, 2009, which claims priority to U.S.Provisional Application No. 61/115,136, filed Nov. 17, 2008 the entirecontents of each of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to a coordinate measuring device. One setof coordinate measurement devices belongs to a class of instruments thatmeasure the three-dimensional (3D) coordinates of a point by sending alaser beam to the point. The laser beam may impinge directly on thepoint or may impinge on a retroreflector target that is in contact withthe point. In either case, the instrument determines the coordinates ofthe point by measuring the distance and the two angles to the target.The distance is measured with a distance-measuring device such as anabsolute distance meter or an interferometer. The angles are measuredwith an angle-measuring device such as an angular encoder. A gimbaledbeam-steering mechanism within the instrument directs the laser beam tothe point of interest. Exemplary systems for determining coordinates ofa point are described by U.S. Pat. No. 4,790,651 to Brown et al. andU.S. Pat. No. 4,714,339 to Lau et al.

The laser tracker is a particular type of coordinate-measuring devicethat tracks the retroreflector target with one or more laser beams itemits. A coordinate-measuring device that is closely related to thelaser tracker is the laser scanner. The laser scanner steps one or morelaser beams to points on a diffuse surface.

Ordinarily the laser tracker sends a laser beam to a retroreflectortarget. A common type of retroreflector target is the sphericallymounted retroreflector (SMR), which comprises a cube-cornerretroreflector embedded within a metal sphere. The cube-cornerretroreflector comprises three mutually perpendicular mirrors. Thevertex, which is the common point of intersection of the three mirrors,is located at the center of the sphere. Because of this placement of thecube corner within the sphere, the perpendicular distance from thevertex to any surface on which the SMR rests remains constant, even asthe SMR is rotated. Consequently, the laser tracker can measure the 3Dcoordinates of a surface by following the position of an SMR as it ismoved over the surface. Stating this another way, the laser trackerneeds to measure only three degrees of freedom (one radial distance andtwo angles) to fully characterize the 3D coordinates of a surface.

Yet there are measurements in which six, rather than just three, degreesof freedom are needed. Here are examples of four such measurements: (1)a six degree-of-freedom (6 DOF) tracker measures the location of a probetip that is blocked from the view of the tracker by an intermediateobject; (2) a 6 DOF tracker follows the motion of a scanning device thatmeasures 3D coordinates using a pattern of light; (3) a 6 DOF trackerfinds the orientation, as well as position, of a robot end effector orsimilar rigid body; and (4) a 6 DOF tracker measures fine objectfeatures using a fine probe tip rather than the large spherical surfaceof an SMR.

Several systems based on laser trackers are available or have beenproposed for measuring six degrees of freedom. In one system, a cameraand laser tracker are used with a target containing a retroreflector andmultiple points of light. Exemplary systems are described by U.S. Pat.No. 5,973,788 to Pettersen et al. and U.S. Pat. No. 6,166,809 toPettersen et al.

In a second system, the target is kept nearly perpendicular to thetracker laser beam by means of motorized or hand adjustment. A beamsplitter in the target sends some of the incoming laser light to aposition detector, which determines pitch and yaw angles of the target.The rest of the light goes to a retroreflector. Of the reflected light,some passes to a polarizing beam splitter, detectors, and electronics,which determine the target roll angle. The remaining light returns tothe tracker. An exemplary system is described by U.S. Pat. No. 7,230,689to Lau.

A third system is the same as the second system except that the rollsensor is replaced by level sensor that measures the tilt of the targetrelative to gravity. An exemplary system is described in U.S. Pat. No.7,230,689 to Lau.

In a fourth system, the tracking device measures the position of acube-corner retroreflector while also splitting off some of thereturning light and sending it to a photosensitive array for analysis.The photosensitive array reads marks intentionally placed on theretroreflector. These marks may, for example, be the intersection linesof the three cube-corner reflection planes. The pitch, yaw, and rollangles of the retroreflector are found by analyzing the patterndisplayed on the array. An exemplary system is described in U.S. Pat.No. 5,267,014 to Prenninger.

In a fifth system, an aperture is cut into the vertex of the cube-cornerretroreflector. Light passing through the aperture strikes a positiondetector, thereby providing pitch and yaw angles of the target. The rollis found by one of three means. In the first means, a camera mounted onthe tracker measures illuminated points of light in the vicinity of theretroreflector. In the second means, a light source mounted on thetracker emits light over a relatively wide angle, which is picked up byposition detector. In the third means, a light source mounted on thetracker projects a laser stripe onto the target. The stripe is picked upby one or more linear arrays. An exemplary embodiment is described inU.S. Pat. No. 7,312,862 to Zumbrunn et al.

Each of these systems of obtaining 6 degrees of freedom (DOF) with alaser tracker has shortcomings. The first system uses a camera to viewmultiple LEDs in the vicinity of a retroreflector target. A commercialsystem of this type available today has a camera mounted on top of atracker. A motorized zenith axis tilts the camera and motorized zoomlens focuses the spots of light. These motorized features arecomplicated and expensive.

In some implementations of the second system, a two-axis mechanicalservo mechanism keeps the target pointing back at the tracker. In otherimplementations, the user manually points the target toward the tracker.In the first instance, the implementation is complicated and expensiveand, in the second instance, the implementation is inconvenient for theuser. In addition, the second system uses a polarizing beam splitter,which must be perpendicular to the laser beam for high polarizationcontrast. For this reason, performance tends to degrade in a handheldsystem.

In the third system, level sensors respond to tilt (a gravity effect)and acceleration in the same way. Consequently, when a tilt sensor isplaced in a hand-held probe, the resulting accelerations caused by handmovement can be mis-interpreted as sensor tilt. To get around thisproblem, the manufacturers of level sensors sometimes add dampingmechanisms (such as damping fluid) to slow the response. Such a dampedtilt sensor responds sluggishly to changes in roll angle, which isundesirable.

The fourth system, which reflects light directly from a beam splitter toa photosensitive array to view lines on a cube corner, is limited in itsdepth of field before the line images on the array become blurry anddistorted.

The fifth system requires that an aperture be cut into theretroreflector, thereby degrading retroreflector performance somewhat.It places a position detector, which may be a photosensitive array or aposition sensitive detector (PSD), behind the aperture. This aperture isonly moderately accurate in the case of the PSD and relatively slow inthe case of the photosensitive array. In addition, the system mounts oneof three additional means on the tracker. All three means, as describedabove, are complicated and expensive.

In view of these limitations, there is a need today for a laser-trackerbased 6 DOF measuring system that is simple, inexpensive, and accurate.

SUMMARY OF THE INVENTION

At least an embodiment of a laser tracker system for measuring sixdegrees of freedom, the system may include a tracking unit comprisingand a target. The tracking target may include a payload assemblyrotatable around at least one axis. The payload assembly may include amain optics assembly structured to emit a first laser beam and a patternprojector assembly structured to emit a second laser beam shaped into atwo-dimensional pattern. The target may include a retroreflector and aposition sensor assembly provided proximate to the retroreflector. Acenter of symmetry of the retroreflector is provided on a differentplane than a plane of the position sensor assembly.

At least an embodiment of a pattern projector assembly for use in alaser tracker system for measuring six degrees of freedom may include alaser structured to emit a laser beam, a beam expander structured toexpand the second laser beam, and a shaping element structured to shapethe expanded second laser beam into a two-dimensional pattern.

At least an embodiment of a target for use with a laser tracker systemfor measuring six degrees of freedom may include a retroreflector and aposition sensor assembly provided proximate to the retroreflector. Acenter of symmetry of the retroreflector is provided on a differentplane than a plane of the position sensor assembly.

At least an embodiment of a method of measuring orientation of a targetmay include providing the target having a retroreflector and a positionsensor assembly provided proximate to the retroreflector, wherein acenter of symmetry of the retroreflector is provided on a differentplane than a plane of the position sensor assembly; illuminating thetarget with a laser beam shaped a two-dimensional pattern; recording aposition of the two-dimensional pattern on the position sensor assemblyto create a measured signature value of the pattern orientation;iteratively comparing the measured signature value with a theoreticalsignature value; and calculating an orientation of the target from themeasured signature value when a difference between the measuredsignature value and the theoretical signature value satisfies aconvergence criteria.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, exemplary embodiments are shown whichshould not be construed to be limiting regarding the entire scope of thedisclosure, and wherein the elements are numbered alike in severalFIGURES:

FIG. 1 is a perspective view of an exemplary six degree of freedomtracking system; and

FIG. 2 is an exploded view of an exemplary tracking unit; and

FIG. 3 is a cross-section view of an exemplary tracking unit; and

FIG. 4 is a block diagram of an exemplary payload assembly; and

FIG. 5 is a perspective view of an exemplary payload assembly andexemplary target; and

FIG. 6 is a top view of an exemplary pattern projector; and

FIG. 7 is a side view of an exemplary pattern projector; and

FIG. 8 is an exemplary transmittance pattern of an exemplary apodizer;and

FIG. 9 is a three-dimensional plot showing exemplary irradiance patternof a propagating laser beam following an apodizer; and

FIG. 10 is a three-dimensional plot showing an exemplary irradiancepattern of propagating laser beam after traveling 30 meters; and

FIGS. 11A and 11B are top and side schematic views of components of anexemplary target for angles of incidence of 0 and 45 degrees,respectively; and

FIGS. 12A and 12B are top and side schematic views of components of anexemplary target for angles of incidence of 0 and 45 degrees,respectively; and

FIGS. 13A-13K are top schematic views of components of an exemplarytarget for angles of incidence of 45 degrees with tilt directionsvarying in 10 degree increments from entirely yaw angle to entirelypitch angle.

FIG. 14 is a graph showing errors characteristic of one possibleembodiment.

FIG. 15 shows an exemplary method of calculating the signature of alaser patter on position detectors.

FIG. 16 shows an exemplary iterative process to calculate a position ofa probe tip.

FIG. 17 shows a perspective view of a laser tracker system having atleast two camera assemblies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an exemplary six degree of freedom (6 DOF) trackingsystem 1000 may comprise tracking unit 100, target 400, powersupply/control unit 10, and computer 20. Six degrees of freedom may bethe x, y, z coordinates, and the pitch, roll, and yaw angles of target400 for example.

Tracking unit 100 may comprise azimuth assembly 110, zenith assembly140, and payload assembly 170. Azimuth assembly 110 is stationary withrespect to the stand to which it is mounted. Zenith assembly 140 rotatesabout azimuth axis 510, and payload assembly 170 rotates about zenithaxis 520. In addition, because payload assembly 170 is mounted to zenithassembly 140, it rotates about azimuth axis 510 as well as zenith axis520.

Power supply/control unit 10 provides power to tracking unit 100 and mayalso provide control and computing functions. Computer 20 may use avariety of software packages to analyze and display data.

Target 400 comprises retroreflector 410, position sensor assembly 430,probe body 450, probe stylus 460, probe tip 470, locator spot 480,electronics (not shown), and battery (not shown). Locator spot 480 isshown in FIG. 5. Position sensor assembly 430 comprises positiondetectors 432 and optical filters 434. Elements of target 400 arerigidly attached to one another. Laser beam 550 emitted by tracking unit100 intersects retroreflector 410 and position detectors 432.

Azimuth and Zenith Assemblies

Details of tracking unit 100 are shown in exploded view in FIG. 2 and incross section in FIG. 3. Azimuth assembly 110 comprises post housing112, azimuth encoder assembly 120, lower and upper azimuth bearings114A, 114B, azimuth motor assembly 125, azimuth slip ring assembly 130,and azimuth circuit boards 135.

The purpose of azimuth encoder assembly 120 is to accurately measure theangle of rotation of yoke housing 142 with respect to the post housing112. Azimuth encoder assembly 120 comprises encoder disk 121 andread-head assembly 122. Encoder disk 121 is attached to the shaft ofyoke housing 142, and read head assembly 122 is attached to stationarypost assembly 110. Read head assembly 122 comprises a circuit board ontowhich one or more read heads are fastened. Laser light sent from readheads reflect off fine grating lines on encoder disk 121. Reflectedlight picked up by detectors on encoder read head(s) is processed tofind the angle of the rotating encoder disk in relation to the fixedread heads.

Azimuth motor assembly 125 comprises azimuth motor rotor 126 and azimuthmotor stator 127. Azimuth motor rotor comprises permanent magnetsattached directly to the shaft of yoke housing 142. Azimuth motor stator127 comprises field windings that generate a prescribed magnetic field.This magnetic field interacts with the magnets of azimuth motor rotor126 to produce the desired rotary motion. Azimuth motor stator 127 isattached to post frame 112.

Azimuth circuit boards 135 represent one or more circuit boards thatprovide electrical functions required by azimuth components such as theencoder and motor. Azimuth slip ring assembly 130 comprises outer part131 and inner part 132. Wire bundle 138 emerges from powersupply/control unit 10 and may carry power to the tracker or signals toand from the tracker. Some of the wires of wire bundle 138 may bedirected to connectors on circuit boards. In the example shown in FIG.3, wires are routed to azimuth circuit boards 135, encoder read headassembly 122, and azimuth motor assembly 125. Other wires are routed toinner part 132 of slip ring assembly 130. Inner part 132 is attached topost assembly 110 and consequently remains stationary. Outer part 131 isattached to yoke assembly 140 and consequently rotates with respect toinner part 132. Slip ring assembly 130 is designed to permit lowimpedance electrical contact as outer part 131 rotates with respect tothe inner part 132.

Zenith assembly 140 comprises yoke housing 142, zenith encoder assembly150, left and right zenith bearings 144A, 144B, zenith motor assembly155, zenith slip ring assembly 160, and zenith circuit board 165.

The purpose of zenith encoder assembly 150 is to accurately measure theangle of rotation of payload frame 172 with respect to yoke housing 142.Zenith encoder assembly 150 comprises zenith encoder disk 151 and zenithread-head assembly 152. Encoder disk 151 is attached to payload housing142, and read head assembly 152 is attached to yoke housing 142. Zenithread head assembly 152 comprises a circuit board onto which one or moreread heads are fastened. Laser light sent from read heads reflect offfine grating lines on encoder disk 151. Reflected light picked up bydetectors on encoder read head(s) is processed to find the angle of therotating encoder disk in relation to the fixed read heads.

Zenith motor assembly 155 comprises zenith motor rotor 156 and zenithmotor stator 157. Zenith motor rotor 156 comprises permanent magnetsattached directly to the shaft of payload frame 172. Zenith motor stator157 comprises field windings that generate a prescribed magnetic field.This magnetic field interacts with the rotor magnets to produce thedesired rotary motion. Zenith motor stator 157 is attached to yoke frame142.

Zenith circuit board 165 represents one or more circuit boards thatprovide electrical functions required by zenith components such as theencoder and motor. Zenith slip ring assembly 160 comprises outer part161 and inner part 162. Wire bundle 168 emerges from azimuth outer slipring 131 and may carry power or signals. Some of the wires of wirebundle 168 may be directed to connectors on circuit board. In theexample shown in FIG. 3, wires are routed to zenith circuit board 165,zenith motor assembly 150, and encoder read head assembly 152. Otherwires are routed to inner part 162 of slip ring assembly 160. Inner part162 is attached to yoke frame 142 and consequently rotates in azimuthangle only, but not in zenith angle. Outer part 161 is attached topayload frame 172 and consequently rotates in both zenith and azimuthangles. Slip ring assembly 160 is designed to permit low impedanceelectrical contact as outer part 161 rotates with respect to the innerpart 162.

Main Optics Assembly

Payload assembly 170 comprises main optics assembly 200 and patternprojector assembly 300, as shown in FIG. 4. Main optics assembly 200comprises electrical modulator 210, laser 215, distance processingelectronics 220, position detector 230, beam splitters 240, 242,dichroic beam splitter 244, and output window 246. Laser light emittedby laser 215 passes through beam splitter 240. This beam splitter may bemade of glass, as shown in FIG. 4, or it may be a fiber-optic beamsplitter. Beam splitter 240 transmits part of the laser light 250 andreflects the remainder. Beam splitter 240 is needed is to pass part ofthe returning (retroreflected) laser light to distance processingelectronics 220. The type of distance-measuring system shown in FIG. 4is an absolute distance meter (ADM) based on intensity modulation andphase-measuring methods. An exemplary ADM of this type is described inU.S. Pat. No. 7,352,446 to Bridges and Hoffer. Alternatively, adifferent type of ADM could be used or the distance meter in main opticsassembly 200 could be an interferometer (IFM) rather than an ADM. In thelatter case, electrical modulator 210 would not be needed and theprocessing electronics would be of a different type. Also, the laserswould be different. For an IFM, the laser would need to be frequencystabilized at a known wavelength and have a long coherence length. Foran ADM, the laser would preferably be capable of modulation atfrequencies of at least a few GHz. It is also possible to combine IFMand ADM in main optics assembly 200. In this case, a suitable beamsplitter would be used to combine the IFM and ADM laser beams on the wayout and separate the laser beams on the way back in.

After passing through beam splitter 240, laser beam 250 travels to beamsplitter 242. This beam splitter transmits most of the laser light (say,85%) and reflects the remainder (say, 15%). The purpose of beam splitter242 is to send part of the returning (retroreflected) laser light toposition detector 230 for reasons explained below. Laser beam 250travels to dichroic beam splitter 244 and passes through to outputwindow 246 by which it exits tracking unit 100. The purpose of dichroicbeam splitter 244 is to permit laser beam 250 to be combined with laserbeam 370 generated in stripe projection assembly 300 on the way out oftracking unit 100. Dichroic beam splitter 244 is made of glass and iscoated, preferably with multiple layers of thin film dielectricmaterial, to enable transmission of some wavelengths and reflection ofother wavelengths. For example, if laser 215 is a distributed feedback(DFB) laser of wavelength 1550 nm and laser 315 is a diode laser ofwavelength 635 nm, then dichroic beam splitter 244 would be coated totransmit 1550 nm laser light and reflect 635 nm laser light.

Laser beam 550 that passes out of tracking unit 100 is a combination oflaser beams 250 and 370. Laser beam 250 strikes retroreflector 410. Itis desirable to minimize the size of laser beam 250 over the measurementrange of the tracker in order to reduce clipping of laser beam 250 byretroreflector 410, which may be a cube-corner retroreflector. Tominimize the size of laser beam 250 over the measurement range, theprofile of the laser beam is shaped as nearly as possible to a Gaussianfunction. This results in the smallest possible divergence angle for thepropagating laser beam.

If laser beam 250 strikes the center of retroreflector 410, the laserbeam retraces its original path to tracking unit 100. If laser beam 250strikes off the center of retroreflector 410, the laser beam reflects tothe other side of retroreflector 410 and returns parallel to, but notcoincident with, outgoing laser beam 250.

When laser light 250 re-enters tracking unit 100 through output window246, it passes through dichroic beam splitter 244 and travels to beamsplitter 242, which reflects some of the return light to positiondetector 230. If laser beam 250 strikes the center of retroreflector410, the returning laser beam strikes the center of position detector230. If returning laser beam strikes off the center of position detector230, the returning laser beam strikes off the center of positiondetector 230 and an error signal is generated. This error signalactivates azimuth motor assembly 125 and zenith motor assembly 155 tosteer laser beam 250 to the center of retroreflector 410. By this means,laser beam 550 from tracking unit 100 is able to follow movements ofretroreflector 410. In other words, laser beam 550 tracks retroreflector410.

Position detector 230 may be a position sensitive detector (PSD).Position sensitive detectors may be of the lateral effect type or thequadrant type. Either may be used, but the lateral effect type producesa voltage output that is more linear with respect to the position of thelaser beam that strikes it. For this reason, the lateral effect type ofPSD is preferred. Alternatively, a photosensitive array, rather than aPSD, may be used. The photosensitive array may be a CCD or CMOS array,for example. These arrays are highly linear and provide very accurateindication of return beam position.

Stripe Generator Assembly

Tracking unit 100 and power supply/control unit 10 are able to measurethree degrees of freedom (DOF) of retroreflector 410 without stripeprojection assembly 300. The three degrees of freedom are distance,azimuth angle, and zenith angle to the target, which can be converted toother coordinates such as x, y, and z. Three degrees of freedom areenough to allow measurement of an object with a symmetrical trackertarget such as an SMR, but they are not enough to find the coordinatesof probe tip 470. To do this, the system needs to measure 6 DOF.

The measurement of 6 DOF is made possible through the combined action ofstripe generator assembly 300, main optics assembly 200, and target 400.As shown in FIGS. 4-7, one possible embodiment of stripe generatorassembly 300 comprises laser 315, beam expander assembly 320, apodizer330, special mirror 340, and camera assembly 350. Beam expander assembly320 comprises negative lens 322 and positive lens 324. Camera assembly350 comprises camera 352 and at least one light emitting diode (LED)354. A hole is cut into special mirror 340 to permit visibility oftarget 400 to camera 352 and at least one LED 354. Laser 315 may bepreferably a visible laser having a power that is in the eye safe rangebut that can be seen when shined against an object. For example, in oneembodiment, this laser is a red diode laser having an output power of 39mW. The laser is selected to have a single-mode transverse mode outputwith a Gaussian profile and good beam quality factor (say, M²<1.1).Laser beam 370 is sent through negative lens 322 and positive lens 324of beam expander 320. The distance between negative lens 322 andpositive lens 324 is adjusted to collimate laser light 370 that emergesfrom positive lens 324. The laser beam may optionally be bent by foldmirrors 360, 362 to make laser pattern projector 300 more compact.

Alternatively, it is possible to use one or more cameras in a differentarrangement than shown in FIG. 4. For example, as shown in FIG. 17, twosmall assemblies 352 could be placed symmetrically or asymmetricallyabout or proximate to the output window 246 on the front of payloadassembly 170, each with at least one LED. Such an arrangement wouldprovide stereoscopic viewing to provide an estimate of distance andangles to target 400. In an alternative embodiment, the cameras can bemounted on yoke housing 142.

Collimated laser light 370 passes through apodizer 330 or other suitableshaping element, which shapes the light into a two-dimensional pattern.For example, in at least one embodiment, the two-dimensional pattern maybe pattern of stripes or other suitable pattern. The apodizer may be acontinuous tone film transparency attached with optical cement betweentwo glass plates. The laser light reaching apodizer 330 has a Gaussianshape, which in one embodiment has a diameter of 44 mm. Thetransmittance characteristics of the apodizer are selected to produce anoptical irradiance (optical power per unit area) at the output of theapodizer having particular characteristics that will now be describedfor one embodiment. The output transmittance of one embodiment of theapodizer is shown in FIG. 8. In the embodiment shown in FIG. 8, apodizeris 38 mm on a side and there are 8 stripes, each of length 15 mm. At thecenter of the apodizer there is a dark area of diameter 8 mm. The shapeof each of the 8 stripes on the apodizer is more or less that of atwo-dimensional Gaussian pattern with the width of the stripe larger inone dimension than the other. The shape is not exactly Gaussian alongthe long dimension of the stripe as a smoothing filter has been appliedto maintain a smooth transition to nearly zero at the edges withoutdecreasing the width of the Gaussian shape too much. It is also possibleto use other apodizer patterns and get good results.

The irradiance of the laser beam that emerges from the apodizer is shownin FIG. 9. As the laser beam propagates, it changes shape because of theeffects of diffraction. The shape of the laser beam at 30 meters isshown in FIG. 10.

It is also possible to generate the stripe pattern using other methods.One way to generate such a pattern is to use a diffractive element. Suchelements are routinely used to produce a variety of patterns includinglines, boxes, circles, and so forth. The pattern may be Gaussian alongthe short axis and nearly Gaussian along the longer axis. This minimizesdivergence of the projected stripes and minimizes the presence ofFresnel diffraction ripples that can bias the calculated centroid orpeak values.

Another way to produce a pattern is to use a collection of suitablelenses. For example, a stripe pattern having a Gaussian cross sectionalprofile can be created by using four cylindrical lenses whose beams arecreated and combined using a series of beam splitters and right angleprisms. The resulting pattern differs from the pattern shown in FIGS. 9and 10 in that the beam does not decrease smoothly to a minimum at thecenter of the pattern. The quality of this pattern can, in principle, bemade nearly as good as that obtained with apodizer 330.

Target

Within target 400, retroreflector 410 may be a cube-corner prism made ofglass. Such a cube corner prism has three perpendicular faces that sharea common point of intersection called the vertex. The top surface of thecube-corner prism is coated with an anti-reflection coating, and thethree perpendicular glass faces are coated with a reflective coating,preferably a multi-layer thin film dielectric coating. It is possible touse a cube-corner prism made, not of solid glass, but of three mirrorsat right angles to one another. This type of retroreflector is oftencalled an open-air cube corner. The advantage of the glass prism overthe open-air cube corner is that the glass bends the laser light inwardin accordance with Snell's law. As a result, a cube-corner prism has agreater acceptance angle than an open-air cube corner. Another advantageof the glass cube corner is that no extra space is needed for the mirrorthickness, which allows position detectors 432 to be closer toretroreflector 410.

The cube-corner prism may be made of high-index glass; for example, anindex of refraction of 1.80 at a wavelength of 1550 nm. On possibleexample of such a glass is Ohara S-TIH53. High-index glass has theadvantage of bending the light that passes from air into glass closer tothe surface normal. Consequently, laser light 250 intersects the frontsurface of retroreflector 410 closer to the center. This reducesclipping of the laser beam by the edges of the cube corner.

It is also possible to use other types of retroreflectors such as acateye retroreflector. The cateye retroreflector is made of glasscomponents of either spherical or hemispherical shape. It is designed sothat laser light entering its front (curved) surface passes throughlayers of glass in such a way as to bring the light to a small spot nearthe back surface. The back surface may be coated to be highly reflectivein order to send the light back on itself. After retracing itself backthrough the glass, the light emerges from the cateye approximatelycollimated and parallel to the incoming beam of light.

Position detector assembly 430 comprises position detectors 432 andoptical filter 434. Position detectors 432 may be linear photosensitivearrays. Such photosensitive arrays may be CCD or CMOS arrays, but CCDarrays are more readily available. In one embodiment, position detectors432 are e2v model TH7815A. In one possible embodiment, these arrayscontain 4096 pixels, each being 10 micrometers on a side. The length ofthe active detector area is 40.96 mm. The height and width of the chippackage, including through leads, are 50 mm and 10.47 mm respectively.

It is possible to use other types of position detectors in place oflinear photosensitive arrays. For example, one could form a linear arrayin the shape of a circle. It would also be possible to use an areaarray.

Optical filter 434 is made up of an optical bandpass filter and optionalneutral density filter. The optical bandpass filter passes only a narrowband of wavelengths (say, 10-20 nm) centered about the wavelength oflaser light 370. Other wavelengths are reflected or absorbed. Thepurpose of the bandpass filter is to prevent undesired background lightfrom illuminating position detector 432 and thereby adding bias andnoise to the measurements. A bandpass filter may be made by coatingglass with a multi-layer stack of thin-film dielectric material. Thereflectance properties of such filters change with the angle ofincidence of the incoming light. The filter can be designed to pass theappropriate wavelengths over the full range of incident angles. Forexample, in one embodiment, the target is capable of operating over+/−45 degrees.

Optical filter 434 may also incorporate a neutral density filter. Asmentioned above, in at least one embodiment the stripe pattern be brightenough to be seen by eye when striking a background object. The brightstripe pattern can help a user quickly find the laser beam if trackingunit 100 is not tracking target 400. Position detectors 30, on the otherhand, need a relatively small amount of laser power; these devicessaturate when the laser power is too high. There are two ways aroundthese conflicting requirements. The first way is to increase the powerof laser beam 370 when laser beam 250 is not tracking on retroreflector410 and then decrease the power of laser beam 370 when tracking begins.The second way is to place neutral density filters over positiondetectors 432 in order to reduce irradiance of laser beam 370 to anappropriate level. This second method has the added advantage ofreducing background radiation relative to the saturation power ofposition detectors 30. One possible way to combine bandpass and neutraldensity functionality in a single filter is to coat neutral densityglass with dielectric film layers to get the desired bandpasscharacteristics.

Another possible way to reduce sensitivity of the system to backgroundlight is to chop the laser beam (by modulating the laser power on andoff at the desired rate) and to detect the laser light at the same rate.This method can provide very high rejection of background light.

There are several possible ways of mounting optical filter 434. Forexample, it may be glued directly to the top of each position detector432, or it may be separated from each photosensitive array by mechanicalmeans. In the latter case, an air gap will exist between optical filters434 and position detectors 432. It is also possible to directly coatposition detectors 432 to provide optical filtering.

Retroreflector 410, position detector assembly 430, and stylus 460 areall rigidly attached to probe body 450. Retroreflector 410 and positiondetectors 432 may be held rigidly by a common structural componenthaving a suitable coefficient of thermal expansion (CTE). Probe body 450can also be attached to this common structural component. Having commonstructural mounting helps reduce mechanical movement from flexing orthermal expansion of printed circuit board material.

Locator spot 480, which is shown in FIG. 5, may be a photogrammetrictarget illuminated by at least one LED 354, or it can be a point sourceof light such as an LED. The purpose of locator spot 480 is to provide acorrespondence between each of the stripes of laser beam 370 and theregions of intersection of the stripes on position sensors 432A, 432B.When tracking unit 100 is operating in tracking mode, laser beam 550 iskept centered on camera 352. Locator spot 480 is found on the camera ata position that corresponds to the target orientation; for example, itis located below the center of camera 352 if target 400 is in theupright position.

Several devices may be used as alternatives to locator spot 480 toidentify the stripes that intersect position detectors 432A, 432B. Onealternative device is a mechanical beam blocker that selectivelyprevents light from reaching the various stripes within the patternprojector assembly 300. Another alternative is a tilt sensor locatedwithin target 400 and tracking unit 100. The relative tilt of target 400to tracking unit 100 identifies each stripe.

Light that intersects position detectors 432A, 432B is converted into anelectrical signal by the detectors and must be processed electrically tofind the peak or centroid of the intersecting stripes. It must befurther processed to find the yaw, pitch, and roll angles of target 400and coordinates of probe tip 470. This processing may be done byelectronics on target 400 or they may be relayed by wired or wirelessmeans back to tracking unit 100, power supply/control unit 10, orcomputer 20 for processing.

Measurement Concept

FIG. 11A shows front and side views of laser beam 250 striking cubecorner retroreflector 410 at normal incidence and laser beam 370striking position sensors 432A, 432B at normal incidence. Laser beam 250strikes vertex 414 and also center 412 of the top face of cube cornerretroreflector 410. Laser beam 250 is kept centered at vertex 414 by thecombined action of position detector 230 and motor assemblies 125, 155.Laser beam 370 is always coincident with laser beam 250. In theparticular case shown in FIG. 11A, the stripes of laser beam 370intersect active area 433A of position detector 432A in three regionsand active area 433B of rightmost detector 432B in three regions. Thenumber of points of intersection depends on the roll angle of target 400relative to tracking unit 100, as is explained in more detail below.

FIG. 11B shows front and side views of laser beam 250 striking cubecorner retroreflector 410 at 45 degrees from normal incidence and laserbeam 370 striking position sensors 432A, 432B at 45 degrees from normalincidence. Laser beam 250 bends inward toward the surface normal when itpasses from air into the glass. In this case, the index of refraction ofthe glass is 1.8, and so by Snell's law the angle of the laser beam withrespect to the surface normal is)arcsin(sin(45°/1.8)=23.1°. Becauselaser beam 370 strikes position detectors 432A, 432B at an angle of 45degrees, the pattern of laser stripes when viewed from the top is anellipse rather than a circle. If stripes within laser beam 370 areextended along straight lines, they intersect top surface ofretroreflector 410 at point 416. As laser beams 250 and 370 are tiltedfurther from the surface normal, point of intersection 416 moves furtheraway from center 412 of the top surface of retroreflector 410.

It is the movement of the intersection point 416 away from center point412 that makes it possible to find the pitch and yaw of target 400. Thismovement can occur if the center of symmetry of retroreflector 410 islocated off the plane of position sensors 432A, 432B. In the case of thecube corner retroreflector, the vertex is always located below the topsurface of the retroreflector, and so this condition is met for theconfiguration shown in FIGS. 11A and 11B.

FIGS. 12A and 12B show target 1400, an alternative embodiment that isthe same as target 400 except that retroreflector 410 is raised somewhatabove the plane of position detectors 432A, 432B. Laser beam 250 strikesthe top surface of cube corner retroreflector at the same location 416relative to center 412 as in target 400. However, as can be seen fromFIG. 12B, the stripes of laser beam 370 intersect active areas 433A,433B in different regions than in target 400. Also the effective pointof intersection 416 has moved. The side view of FIG. 12B shows that theeffective point of intersection 416 can be found by extending laser beam250 along its original air path into the glass. The point at which thecenter of laser beam 250 intersects the plane of position detectors432A, 432B is the effective point of intersection 416 when viewed fromthe top.

The effect of raising retroreflector 410 above the plane of positiondetectors 432A, 432B is to cause the point of intersection 416, as seenfrom the top view, to move closer to center 412. Another effect is toreduce the portion of the stripes that intersect position detectors432A, 432B. Retroreflector 410 may be raised as high as desired as longas the stripes of laser beam 370 are not occluded from reaching positiondetectors 432A, 432B.

By raising retroreflector 410 above the plane of position detectors432A, 432B, it is possible, using commercially available linear CCDarrays, to measure pitch and yaw angles over the range of 0 to 45degrees. This capability is demonstrated in FIG. 13. The small inset atthe top right of FIG. 13 shows the conventions used for pitch, roll, andyaw angles. The x axis is has the same direction as position detectors432A, 432B. A yaw angle corresponds to a rotation about the x axis.Pitch and roll angles correspond to rotations about the y and z axes,respectively.

FIGS. 11B and 12B show the effect of yaw rotation for a pitch angle ofzero. As target 400 is yawed to the left, the stripe separationincreases on position detector 433A and decreases on position detector433B. If target 400 were rotated in pitch angle, the stripes of laserbeam 370 would move up or down on position detectors 432A, 432B. Iftarget 400 were rotated in the roll angle, the stripes of laser beam 370would rotate about intersection point 416. Camera assembly 350 andlocator spot 480 are used to draw a correspondence between each stripegenerated by stripe generator 300 and each region of stripe intersectionon position detectors 432A, 432B.

FIGS. 13A-K show the general case. Here the overall tilt is 45 degrees.The direction of tilt is the direction from center 412 to point ofintersection 416. The stripes are stretched into an elliptical patternhaving a major (long) axis along the direction of tilt. A laser stripedoes not necessarily lie on the long axis. The tilt is entirely a yawangle if the direction of tilt is perpendicular to position detectors432A, 432B. The tilt is entirely a pitch angle if the direction of tiltis parallel to position detectors 432A, 432B. The tilt is a combinationof yaw and pitch for other cases. The roll angle is indicated by theorientation of each stripe with respect to some reference and is basedpartly on the information provided by locator spot 480.

FIG. 13A shows the case in which target 400 has a yaw angle of 45degrees and a pitch angle of zero (relative to the incoming laser beam).Laser beam 370 approaches target 400 from the right in this instanceand, because of this, the major axis of the ellipse that encompasses thestripes is along the y (horizontal) axis. Note that the stripesintersect position detector 432A in 3 regions and position detector 432Bin 3 regions. FIG. 13K shows the case in which target 400 has a pitchangle of 45 degrees and a yaw angle of zero. Laser beam 370 approachestarget 400 from the top so that the major axis of the ellipse is alongthe z (vertical) axis. Stripes still intersect position each positiondetector 432A, 432B in 3 regions, but now the stripes are shorter and sothe regions of intersection occur nearer the ends of the stripes.

In moving from FIG. 13A to FIG. 13K, the direction of the tilt changesby 10 degrees per step. This series of figures shows that there arealways at least two pairs of opposing stripes that point unambiguouslyto a single spot 416. This condition is sufficient to find the pitch andyaw angles of target 400.

Measurement Method

The measurement concept described so far explains the general method andapparatus that enables measurement of six degrees of freedom. somepossible computational methods that can be used will be described.

Defined are three angles phi, theta, and roll that fully constrain theposition of the target relative to the laser beam coming from the lasertracker. First the z axis is defined as the axis perpendicular to theplane that holds position detectors 432A, 432B, and the x axis isdefined as along the direction of position detectors 432A, 432B. Theyaxis is perpendicular to the x and z axes. The angles theta and phi aredefined in the usual way with respect to the laser beam in a sphericalcoordinate system. Theta is the angle from the z axis to the laser beam,and phi is the angle from the x axis to the projection of the laser beamonto the xy plane. By convention, it is assumed that the laser beam hasa phi of 0 degrees if it arrives from the top of a two dimensionalfigure such as FIGS. 11-13. A phi of +90, +180, and +270 degreesindicates arrival of the laser beam from the right, bottom, and left,respectively. The roll angle is taken with respect to a particularreference stripe emitted by the laser tracker. For example, suppose thatprobe 400 and tracking unit 100 are oriented as shown in FIG. 1 and alsosuppose that the laser stripes are emitted in the orientation shown inFIG. 8. If the apodizer stripe in the upper right side of FIG. 8 isselected as the reference stripe, then a roll angle for that stripe canbe established in relation to the angle phi=0 degrees. In this case,since the reference stripe is rotated 22.5 degrees with respect to phi=0degrees, one could say that the roll angle for the light projected fromthe apodizer is 22.5 degrees. If the probe were tilted by 45 degreeswith respect to this initial position, the new roll angle would be22.5+45 degrees=67.5 degrees. In this way, the roll angle can take onany value from 0 to 360 degrees.

Because the shape of the laser beam evolves as it propagates, as shownin FIGS. 9 and 10, it is necessary to account for the change in theintersection pattern of the stripes with position detectors 432A, 432B.Furthermore, the stripes are not perfectly uniform, and so the exactpattern of intersection of the laser beam with the stripes depends onthe exact shape of the stripes. In practice, therefore, one may samplethe patterns observed on position detectors 432A, 432B at differentdistances and different angles of tilt (phi, theta, and roll).

To optimize accuracy of the measurement, probe tip 470 may be placeddirectly beneath retroreflector 410. A numerical analysis was carriedout based on the signal-to-noise ratio of position detectors 432A, 432Band the variability of projected laser pattern 370. FIG. 14 shows theresulting errors in micrometers for angles of phi and theta from 0 to 45degrees. The maximum error in this case is 28 micrometers. This smallerror is acceptable at this range.

Computational Method

FIG. 15 shows an exemplary method of calculating the signature of thelaser pattern on position detectors 432A, 432B. The signature is definedas the position of up to six peaks and valleys on the two positiondetectors. Each of these peaks is associated with a particular stripeand each of the valleys falls between two peaks. The signature alsoprovides a subpixel location for the intersection of the center of eachpeak or valley on the array.

The first several computational steps are the same on first positiondetector 432A and second position detector 432B. These steps areaccumulate, lowpass, decimate, take derivative, find zero crossing, dropsmall peaks-valleys, and fit parabola. These computations may be carriedout with a field-programmable gate array (FPGA), digital signalprocessor (DSP), microprocessor, or computer.

Each position sensor is illuminated by a laser beam, power and dutycycle of which may be adjustable, and for a particular integration time,which may also be adjustable. The adjustment in laser power or dutycycle takes place within the laser tracker. The adjustment inintegration time takes place within the linear array by adjusting the“electronic shutter time”. In either case, the objective is to provideenough light for enough time to obtain a good signal to noise ratiowithout saturating the detectors.

Each set of pixel samples are collected at high speed. The samples areaccumulated, as shown in FIG. 15, by collecting the set of pixel valuessome number of times and averaging these together. As an example, 4096pixel values from position detectors 432A, 432B may be collected at 1600frames per second and averaged in groups of 8 frames to give aneffective data collection rate for the 4096 pixels of 200 Hz.

The accumulated data is next filtered and decimated. Both processes canbe carried out together by a digital filter. The simplest type offiltering averages some number of adjacent channels, but many filteringmethods are available. The decimation removes some number of samples tosimplify computation in later stages. As an example, the data may bedecimated to one eighth the original number of data points.

To extract peaks and valleys from the data, four steps are taken. Firstthe differences (derivatives) are taken between adjacent pixels. Second,the data is analyzed to find the zero crossings. These zero crossingsrepresent the potential peaks and valleys. Third, peaks and valleys thatare too small are dropped. These peaks and valleys may be very noisy orthey may just be too small to be interesting. Fourth, a parabola is fitto the data near the peak or valley. This establishes the location ofthe peaks and valleys to subpixel resolution.

The position of locator spot 480 is used to get an approximate rollangle as a starting position for later calculations. The position oflocator spot 480 is not known well enough on camera 352 to establish theexact roll angle.

The parabola peaks and valleys from the two position detectors areprovided, along with the approximate roll angle from locator spot 480,and this information is used by the computing device to match each peakon position detectors 432A, 432B to a particular laser stripe.

The resulting signature comprises the match of each stripe and thesubpixel values for position detectors 432A, 432B. As shown in FIG. 16,the measured signature is provided to a COMPARE computing function. TheCOMPARE computing function compares the measured and theoreticalsignature values according to a method that is now explained.

The entire iterative computation shown in FIG. 16 is seeded with phi,theta, and roll values, which are provided from a previous calculation.This previous calculation may obtained from the last measuredorientation of probe 400 or provided by an initial approximatecalculation.

A FIT routine shown in FIG. 16 now performs iterative adjustments(typically two or three iterations) to find the values of theta, phi,and roll that result in the best match between the measured andtheoretical signatures. The new phi, theta, and roll are sent to thespline table shown in FIG. 16, along with the known distance to thetarget as measured by the laser tracker. In addition, compensation datathat provides geometrical relations of the detectors and retroreflectoron target 400 are sent to the spline table. From this data, aninterpolated value emerges that gives new more accurate guesses for thepositions of the peaks and valleys. This more accurate guess is calledthe theoretical signature.

The procedure is continued until the difference in the measured andtheoretical signatures is small enough that the convergence criteria ofthe fit routine are satisfied. At this point, the best guess values forphi, theta, and roll are used to calculate the position of the probetip. This calculation takes into account the length and geometry of thestylus and probe in relation to the rest of target 400.

Target Camera

Camera assembly 350 comprises camera 352 and at least one light emittingdiode (LED) 354. As explained above, camera assembly may be used inconjunction with locator spot 480 to identify each of the stripes thatintersects position sensors 432. In addition, camera assembly 350 may beused to enhance operation of the general purpose laser tracker, whethermeasuring three or six degrees of freedom.

For the general purpose laser tracker applications, the LEDs are usuallyflashed repetitively. Light from the LEDs bounces off retroreflectorsand returns to a nearby camera. The camera image shows the normal sceneas well as each of the retroreflectors flashing in unison with the LEDs.Based on this flashing pattern, the operator can quickly learn thenumber and location of retroreflectors.

One advantage of the camera is that it can speed acquisition of targets.However, in trackers available today, the camera (if present) is locatedoff the optical axis of the tracker. The resulting parallax makes itimpossible for the tracker to immediately drive to the correct angle ofthe selected retroreflector.

Camera assembly 350 gets around this problem by mounting camera 352 onthe tracker optical axis or optical axis of the pattern projectorassembly, which eliminates the parallax. Another way to get around theproblem is to use two cameras 352 equally or symmetrically spaced oneither side of or around the tracker optical axis or optical axis of thepattern projector assembly, as seen in FIG. 17. In FIG. 17, the cameras352 are positioned on either side of the output window 246. LEDs 354 areprovided proximate to cameras 352 to complete the camera assembly. Inthis case, triangulation can be used to find the position of the target.In another embodiment, the cameras are not required to be positionedsymmetrically about the optical axis. It is possible to calculate thethree-dimensional position of a retroreflector as long as the positionof the two cameras in the tracker frame of reference and the two anglesmeasured by each of the two cameras are known.

Camera assembly 350 can drive to any desired retroreflector. Theoperator may do this by selecting the desired SMR on a computer screen.Alternatively, the computer may be set up to automatically acquire anSMR whenever it is brought into the field of view. This feature is mostuseful when only one SMR is present.

One common use of targeting cameras is to set up a survey measurement ofa number of retroreflector targets. With current trackers, this is doneby selecting one SMR after another on the computer screen and thensearching with the tracker to find each target. The laser beam may beplaced near to the target at the start of the measurement. With in-linecamera assembly 350, it is possible to automatically and quickly locateeach retroreflector in the environment and automatically create a surveypattern. This can save considerable time, particularly when targets aredifficult to reach. A good example of such time savings is the joiningof two fuselage sections of an airliner. One method of performing thisjoin is to attach a number of small retroreflector targets onto the twofuselage sections, in many cases at locations that are not easy toreach. A completely automated survey greatly simplifies this procedure.

While the description above refers to particular embodiments of thepresent invention, it will be understood that many modifications may bemade without departing from the spirit thereof. The accompanying claimsare intended to cover such modifications as would fall within the truescope and spirit of the present invention.

The presently disclosed embodiments are therefore to be considered inall respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A laser tracker configured to locate aretroreflector target, the laser tracker comprising: a payload assemblyconfigured to emit a first beam of light along a main optical axisextending from the laser tracker; a first motor and a second motorconfigured to together direct the first beam of light along the mainoptical axis in a first direction, the first direction determined by afirst angle of rotation about a first axis and a second angle ofrotation about a second axis, the second axis substantiallyperpendicular to the first axis, the first angle of rotation produced bythe first motor and the second angle of rotation produced by the secondmotor, wherein the payload assembly is configured to rotate about thefirst axis and the second axis; a first angle measuring deviceconfigured to measure the first angle of rotation and a second anglemeasuring device configured to measure the second angle of rotation; adistance meter configured to measure a first distance from the lasertracker to the retroreflector target; a first camera assembly includinga first camera and a first light source, the first camera assemblyrigidly affixed to the payload assembly, the first camera having a firstcamera optical axis extending from the payload assembly, the firstcamera optical axis generally in the same direction as the main opticalaxis and offset with respect to the main optical axis, the first lightsource proximate to the first camera, the first light source configuredto illuminate the retroreflector target to produce a firstretroreflected light, the first camera configured receive the firstretroreflected light and to produce a first image; and a second cameraassembly including a second camera and a second light source, the secondcamera assembly rigidly affixed to the payload assembly, the secondcamera having a second camera optical axis extending from the payload,the second camera optical axis offset generally in the same direction asthe main optical axis and offset with respect to the main optical axisand to the first camera optical axis, the second light source proximateto the second camera, the second light source configured to illuminatethe retroreflector target to produce a second retroreflected light, thesecond camera configured receive the second retroreflected light and toproduce a second image.
 2. The laser tracker of claim 1 wherein thefirst camera optical axis and the second camera optical axis are placedsymmetrically about the main optical axis.
 3. A method for locating aretroreflector target with a laser tracker, the method comprising:providing a payload assembly configured to emit a first beam of lightalong a main optical axis extending from the laser tracker; providing afirst motor configured to rotate the payload about a first axis by afirst angle of rotation; providing a second motor configured to rotatethe payload about a second axis by a second angle of rotation, whereinthe second axis is substantially perpendicular to the first axis;providing a first camera assembly including a first camera and a firstlight source, the first camera assembly rigidly affixed to the payload,the first camera having a first camera optical axis extending from thepayload, the first camera optical axis generally in the same directionas the main optical axis and offset with respect to the main opticalaxis, the first light source proximate to the first camera; providing asecond camera assembly including a second camera and a second lightsource, the second camera assembly rigidly affixed to the payload, thesecond camera having a second camera optical axis extending from thepayload, the second camera optical axis offset generally in the samedirection as the main optical axis and offset with respect to the mainoptical axis, the second camera optical axis offset generally withrespect to the first camera optical axis, the second light sourceproximate to the second camera; emitting the first beam of light;illuminating the retroreflector target with a third light from the firstlight source to produce first retroreflected light from theretroreflector target; illuminating the retroreflector target with afourth light from the second light source to produce secondretroreflected light from the retroreflector target; receiving the firstretroreflected light by the first camera and producing a first cameraimage in response; receiving the second retroreflected light by thesecond camera and producing a second camera image in response;determining a first calculated angle of rotation and a second calculatedangle of rotation to steer the main optical axis toward theretroreflector target, the first calculated angle of rotation and thesecond calculated angle of rotation based at least in part on the firstcamera image and the second camera image; and rotating the payload bythe first calculated angle of rotation about the first axis and thesecond calculated angle of rotation about the second axis.
 4. The methodof claim 3 further comprising: providing a position detector; receivingthe first beam of light at the retroreflector target and returning athird retroreflected beam of light in response; receiving a portion ofthe third retroreflected beam of light at the position detector; androtating the payload with the first motor and the second motor to placethe third retroreflected beam of light in a preferred position on theposition detector.
 5. The method of claim 4 further comprising:providing a first angle measuring device, a second angle measuringdevice, and a distance meter; measuring a third angle with the firstangle measuring device; measuring a fourth angle with the second anglemeasuring device; measuring a first distance from the laser tracker tothe retroreflector target based at least in part on a time for the firstbeam of light to travel from the payload to the retroreflector target;and providing a three-dimensional coordinate of the retroreflectortarget based at least in part on the third angle, the fourth angle, andthe first distance.
 6. The method of claim 3 wherein: in the step ofilluminating the retroreflector target with a third light, the thirdlight is flashed repetitively; and in the step of illuminating theretroreflector target with the fourth light, the fourth light is flashedrepetitively.
 7. The method of claim 3 wherein: in the step of providinga first camera assembly, the first light source includes a first LED;and in the step of providing a second camera assembly, the second lightsource includes a second LED.
 8. The method of claim 3 wherein, in thesteps of providing the first camera assembly and the second cameraassembly, the first camera optical axis and the second camera opticalaxis are placed substantially symmetrically about the main optical axis.